With the recent development of technology, in addition to electric vehicle applications, a large-scale lithium ion secondary battery for the purpose of electric energy storage has been put to practical use. These large-scale storage batteries for electric energy storage are installed in houses and offices or in premises of electric power companies, and the term of operation thereof is assumed to be 10-year period or longer. Those used for such applications are also referred to stationary storage batteries, and are distinguished from storage batteries mounted in electronic devices, vehicles or the like.
For the large-scale lithium ion secondary battery expected to be used for 10-year or longer, it is necessary to ascertain the state of battery as maintenance and management. For example, continuous monitoring of the temperature is important for judging abnormalities such as heat generation. In addition, the assessment of the available battery capacity (also referred to as chargeable storage capacity) is required for judging necessity of a maintenance work of the battery.
Incidentally, the deterioration of lithium ion secondary batteries progresses with the use thereof, and it is known that an available battery capacity decreases. In case that the battery capacity decreases faster than expected due to certain abnormality, or in case that it decreases below a guaranteed capacity within a warranty period, maintenance and replacement is necessary.
If the timing of these maintenance and replacement can be scheduled in advance, the arrangement of servicepersons and procurement of spare parts can be carried out in planed manner. If this is realized, the operation stopping period of the storage battery can be scheduled, and therefore inconvenience to users is reduced and satisfaction of users will be improved. The systematic procurement of parts allows to minimize the stock thereof and to reduce unnecessary cost.
One of the simplest methods to ascertain a chargeable storage capacity is discharging a storage battery completely after charging it to fully charged state (see also paragraph [0005] etc. of Patent Document 1 although it is for vehicles), and determining the capacity during the discharge.
However, depending on a user, the complete discharge of a battery seldom takes place. This is because, for example, there are users who want to keep an electric energy more than SOC 30% all the time. In addition, depending on a user, because of the low use of electric energy during the daytime, there is a case that a complete discharge is not attained. In these cases, it is technically possible to forcibly discharge by some ways, for example, such a way as to suppress the charging for a certain period of time. However, frequent forced operations which are irrelevant to user's intention restrict the use of the user, impairing the convenience.
Then, as another method for ascertaining the battery capacity, also conceivable method is to analyze the log data of the lithium-ion secondary battery and estimates the amount of deterioration.
However, in this case, it is necessary to obtain the charge and discharge curve with high accuracy to some extent, and it is desirable to charge after fully discharged even in this method. Therefore, this method also has a problem to impose restrictions on the use of the user.
Besides these, there is also a method of estimating the amount of reduction in battery capacity by measuring electric impedance or resistance. However, such an approach requires a measuring mechanism other than the mechanisms for the measurement of current, voltage and electric power, leading to increase in cost.
As seen above, there is a need for ascertaining a chargeable storage capacity of a storage battery in a manner that does not cause the increase in cost and does not impair convenience for users.